Remove no longer needed --keep-file-descriptors option

We added this option in commit d17b252, but Bundler now keeps
descriptors by default. While this behavior was backported to Ruby
3.0.x, we're changing it now because, we've only noticed it now that
we're upgrading to Ruby 3.1.x, since it was first developed for that
version [1].

[1] rubygems/rubygems@88b7a3e7e2

config/puma/defaults.rb

@@ -18,7 +18,6 @@
 workers 2
 preload_app!
 
-restart_command "bundle exec --keep-file-descriptors puma"
 plugin :tmp_restart
 
 on_restart do